Pincer ligands are chelating agents that bind metals tightly to three adjacent coplanar sites. The pincer-metal interaction is rigid and typically confers a high thermal stability to the ligand metal complexes. Organic portions and substituents define a hydrophobic pocket around the coordination site. These ligands traditionally share the common feature of a central aromatic unit. To this central unit are attached, in the ortho positions, two arms whose electronic and steric properties can be varied in many different ways. The ability to vary the properties of pincer ligands has been exploited for numerous complexes to be used as catalysts. Early work mainly focused on ligands where the central binding site is carbon and the peripheral binding sites are phosphorous, generally referred to by the atomic symbols of the donor atoms at the binding sites as the PCP systems. More recently CCC, CNC, CNS, NNN, NCN, PNP, OCO, SCS, SNS have been reported. Most frequently the pincer ligand transition metal complexes have been those of group VII-X metals where low coordinate and low oxidation state prevail and the metals are tolerant of a wide variety of substituents.
Early transition metal (group III-VI) pincer complexes are significantly less common and typically display high oxidation states and high coordination numbers, are typically electrophilic, and are intolerant of many functional groups. As most presently known pincer ligands have multiple soft donor atoms for metal binding, the ligands are not well suited to forming complexes with the early transition metals. Those that have been prepared include: pincer dicarbene complexes of CNC ligands with V, Ti, Cr, Mn, and Nb; nontraditional NNN ligands with Zr; NCN ligands with W, Mo, Ti, La, Ta and Mn; and OCO ligands with Ti, Ta, and Mo. The early transition metals form complexes with pincer type ligands where the donors are all considered hard donors. Although OCO pincer ligands form transition metal complexes, the metal-carbon bond is susceptible to degradation via insertion reactions. Hence, pincer ligands that are not readily susceptible to degradation but can bind to group III through group X transition metals could be useful for catalysts for a broad scope of reactions including N-atom transfer reactions, aerobic oxidation, olefin polymerization, alkene isomerization, and C—H bond activation.